Such a pump conventionally comprises a pump head that defines a pump chamber, i.e. a chamber of volume that can be varied by deforming or moving a wall of the head. The chamber is secured to a structure that supports the pump head and that drive members that are coupled to the movable or deformable wall.
When said wall is a piston, the drive members are constituted by a linkage connecting the piston to a crank shaft having a single eccentric. When said wall is a diaphragm, which is a configuration with which the invention is mainly concerned, the drive members are either a direct mechanical transmission of the linkage kind, or else a hydraulic transmission with a working chamber on one side of the fluid proof diaphragm using a hydraulic fluid that is driven to move cyclically by a driving piston that urges the fluid towards the diaphragm under pressure, thereby reducing the volume of the pump chamber (pump delivery), or that sucks it back by moving the diaphragm in the opposite direction, thereby increasing the volume of the pump chamber (pump suction).
The pump head has an admission orifice and a delivery orifice, each provided with at least one check valve so that the fluid flows in one direction only through the pump chamber. The check valves are releasably secured to the pump head by means of valve boxes.
The aggressive nature of the pumped fluid leads to high-grade materials being used for those parts that come into contact with the fluid: stainless steel, special alloys, titanium, etc., which materials are particularly expensive.
Concerning the pump head, the most common shape is a body in the form of a substantially cylindrical ring having, on the outside, locations for receiving the valve boxes, closed at one of its ends by a side plate, and closed at its other end by the diaphragm which is clamped between the cylindrical body and the structure of the pump.
The closure side plate is a part that presents a large area in contact with the pumped fluid. It is also a bulky part, which can be extremely expensive when it is to be made of a material that presents a high degree of chemical inertness.
Proposals have been made to coat the surface of this part that faces towards the pump chamber, either by using a kind of vulcanization (see U.S. Pat. No. 2,753,804), or by applying a kind of skin, e.g. of polytetrafluoroethylene, shaped by molding and held in place by being clipped in the pump chamber (see U.S. Pat. No. 3,000,320).
Neither of those solutions is satisfactory: the first because there does not exist any chemically inert material capable of forming a coating that is securely bonded by adhesive or any other surface adhesion technique against the pressure and suction stresses that exist in a pump chamber, and the second because it is not possible to withstand such stresses merely by clamping a skin to a pump side plate. In addition, with a skin, the shaping of the skin, e.g. by molding, can turn out not to be accurately complementary to the surface of the side plate that is exposed in the pump chamber, or not to be sufficiently regular to ensure that no pockets of gas remain between the side plate and the skin, which pockets expand and contract on each pump cycle, thereby reducing the cylinder capacity of the pump.
The invention seeks to solve the question of passivating this surface that is exposed to the pumped fluid by proposing a coating for said surface by means of a skin that is perfectly coated to the shape of the protected surface so that no damage of mechanical origin or as a function of fatigue over time can affect the protection.